Conventional hot-melt adhesives adapted to be applied to the inner surface of a heat-shrinkable tube contain such resin compositions as EVA (ethylene-vinyl acetate copolymer), EEA (ethylene acrylate copolymer), modified polyolefins, polyamides or fluororesins. Further, tackifiers such as coumarone-indene resins, phenol terpene resins or polybutene resins are added to impart adhesiveness.
On the other hand, bromic or chloric substances are used as flame-retardants. Since bromic flame-retardants ape higher in flame retardancy, they are more frequently used. Such flame-retardants include hexabromobenzene, decabromodiphenyl oxides, ethylene-bis-tetrabromophthalimide, tetrabromo-bisphenol A derivatives and octabromodiphenyl oxides. Inorganic flame-retardants include antimony trioxide, antimony pentoxide, magnesium hydroxide and ammonium polyphosphate.
It is well-known that a high polymeric material shows thixotropy if fine powder is dispersed in the material. This characteristic is widely used for sealing materials and paints. As fine powders to be added to a material to impart thixotropy to the material, silica having an average particle diameter of several microns and hydrate aluminas having an average particle diameter of 1.2 microns or less are known as shown in Japanese Examined Patent Publications 54-37665 and 55-29107. Also, in applications where both flame-retardancy and waterproofness are required, trials have been made to add to such fine powder flame-retardants or substances which show flame retardancy as disclosed e.g. in Japanese Unexamined Patent Publication 61-254686.
Considering the object of a flame-retardant hot-melt adhesive to be applied to an inner layer, it is required that its fluidity increase when heated. But, since the fluidity also increases when burning, the adhesive may flow, increasing the possibility of spreading flames. It is possible to make use of the thixotropy effect in order to increase the apparent viscosity while burning, that is, while the adhesive is subjected to a relatively small external force, and thus to prevent the adhesive from flowing during burning. But if the amount of fine powder added is increased in order to increase the thixotropy effect. high fluidity would not be obtained. This increases the viscosity of the adhesive during waterproof treatment to such an extent that no sufficient packing is possible. Thus, the waterproof properties are bad.